1. Field of Invention
The invention relates to a lens for digital cameras and, in particular, to an auto focus lens system.
2. Related Art
There are many ways to help people memorize happy times. Taking pictures is one of the most frequent ways. Therefore, one often needs to use a camera for recording the memorable moments. As we can see, the designs of cameras have evolved toward compact styles, so have the currently popular digital cameras.
The focus structure of digital cameras can be roughly divided into a manual focus structure and an auto focus structure. The manual focus structure allows one to focus the lens to a desired position, whereas the auto focus structure focuses on an object by comparing with the image on an image sensor. The image sensor is either a charge coupled device (CCD) sensor or a complementary metal oxide semiconductor (CMOS) sensor, and is controlled by a central processing unit (CPU) installed inside the digital camera. In the beginning, the lens moves back and forth around its original position (static position). As the lens moves, the CPU compares the image from the lens with the image in the image sensor. Finally, as the lens moves to the position where the image is the clearest, then the lens stops at the position. In this case, the lens has to be continuously driven back and forth by an annular cam. The annular cam is provided with a lens driving structure and driven to rotate by a stepping motor. At the moment when the CPU detects a clearest image as the lens moves back and forth, a stop signal is simultaneously sent to the lens. Therefore, the lens stops at the best focal position (static position). When the lens is to be moved back to the original position, the annular cam starts to rotate again. Once the lens moves back to its original position, the photo interrupter installed by the lens cylinder uses the shutter at the border of the lens cylinder, to detect the original position of the lens. The shutter will interrupt the light of the penetration-type photo interrupter. The lens stops at the moment when the photo interrupter detects the original position.
Consequently, the lens in a digital camera auto focus system has to be driven continuously, so is the image detection. Every time an image is captured, the lens has to return to its original position. Thus, the lens focusing procedure is very time-consuming, which, in most of cases, results in losing the best image-capturing time. Moreover, the structure including a stepping motor, an annular cam, and a photo interrupter is very expensive.
With reference to the cross-sectional view of a traditional digital camera auto focus structure in FIG. 1 and the corresponding three-dimensional view in FIG. 2, when the stepping motor 1 receives a start signal from the digital camera CPU, it starts to rotate. A stepping motor gear 2 that matches with an annular cam gear 3 provided at the border of the annular cam 4 also rotates. The annular cam 4 thus starts to rotate because of the stepping motor gear 2 and the annular cam gear 3. A position-detecting rod 5 moves back and forth (up and down) along the slope surface of the annular cam 4. The whole lens module 6 moves back and forth along a track 7. One side of the lens cylinder is provided with a sheath, where the position-detecting rod 5 is installed as one part and protrudes from the sheath. The outermost part of the sheath of the lens module 6 is provided with a bearing 8 moving back and forth along the track 7. The other side of the lens is provided with a position-fixing fork 9 as one part of the lens cylinder. A position-fixing pin 10 is provided between the two terminals of the position-fixing fork 9.
The bearing 8, the track 7, the position-fixing fork 9 and the position-fixing pin 10 support the lens module 6. The lens module 6 is always pressed toward an image sensor 11 by a coil spring 12, so that the tip of the position-detecting rod 5 always appropriately touches the slant surface of the annular cam 4.
When the power of the digital camera is turned on, the image is formed on the image sensor 11 by the lens module 6. Once the switch of the auto focus structure is turned on, the image focusing is transmitted to the CPU in a continuous way for checking the image formed on the image sensor 11. When the image becomes clearest, the CPU sends out a stop-checking signal, and the stop-checking signal stops the stepping motor 1, releases the shutter, and captures the image. After these steps, the lens module 6 has to return to its original position (static position). A shutter-detecting blade 13 is provided at the edge of the annular cam 4. After capturing the image, the annular cam 4 starts to rotate again to bring the lens module 6 back to its original position. When the shutter-detecting blade 13 enters the slit of the photo interrupter 14, the beam in the photo interrupter 14 is interrupted by the shutter-detecting blade 13. The annular cam 4 immediately stops here so that the lens module 6 can return and stop at its original position. As shown in FIG. 2, the lens module is right at its original position.
The lens module 6 has to move back and forth for the CPU to check the image. This method of searching for an optimal focal point makes use of mechanical and continuous lens motion to repeatedly check the image. Thus, it is time-consuming so that the user may miss the best timing. Moreover, this complicated structure has to be finely assembled and adjusted. Its components are often very expensive. Therefore, it would be desirable to provide a simple structure that uses fewer cheap components while saving focusing time, electric power, and the required space.